An array antenna is conventionally known as one of directivity antennas. The array antenna has a plurality of arrayed antenna elements, and can electronically change a directivity direction of a beam of a radio wave while controlling a phase of a signal flowing to each antenna element. Since the directivity direction of the beam of the radio wave can be varied by changing a feeding phase of each antenna element, a communication region can be enlarged by scanning the beam of the radio wave as in a tag communication antenna described in Patent Document 1, or use can be made in detection of a tag movement direction as in a tag movement direction detection system described in Patent Document 2. A case in which an angle is denoted with degree (° or deg) as a unit, and a case in which the angle is denoted with a radian as a unit are provided in the present specification and the drawings, where in a portion where the angle is denoted with degree as a unit in a mathematical formula, the angle is handled with degree as a unit in such a mathematical formula. In a portion where the angle is denoted with radian as a unit in a mathematical formula, the angle is handled with radian as a unit in such a mathematical formula
Miniaturization of the array antenna is desired, and reducing the number of configuring antenna elements is the most effective way to miniaturize the array antenna. The applicant uses an array antenna 200 including 3×2=six elements (210a to 210f) of three elements in a horizontal direction (X-axis) and two elements in a vertical direction (Y-axis), as shown in FIG. 7(a), as a prototype. The applicant uses the array antenna 200 as a prototype, and detects the movement direction of a package as described in Patent Document 2. In other words, as shown in FIG. 7(b), the movement direction of a movable body such as a package is detected by changing the feeding phase of each antenna element, and repeatedly changing the directivity direction of a main lobe (MLα, MLβ) or the beam of the radio wave emitted from the array antenna 200 in scan angles α, β (inclination angle in a horizontal direction with respect to a broadside direction). Such a method of detecting the movement direction is described in detail in Patent Document 2, but an outline will be described below with reference to FIG. 7(c).
If the directivity direction of the main lobe is a +direction in the figure with respect to the broadside direction (main lobe MLα), communication is not carried out with the RFID tag attached to the package on the scan angle β side (not shown) and communication is carried out only on the scan angle α side. Similarly, if the directivity direction of the main lobe is a —direction in the figure with respect to the broadside direction (main lobe MLαβ), communication is not carried out with the RFID tag attached to the package on the scan angle α side (not shown) and communication is carried out only on the scan angle β side. Since communication is carried out with the RFID tag by repeatedly switching the directivity direction of the main lobe to the scan angles α, β, a linear approximation line L is obtained from a distribution of a plurality of pieces of data (plot data P) communicated with the main lobe MLα and a plurality of pieces of data (plot data P) communicated with the main lobe MLβ, and a slope thereof is calculated to detect the movement direction. As is apparent with reference to FIG. 7(c), it is important that communication is not carried out with the RFID tag on the − side when switched to the main lobe MLα and that communication is not carried out on the + side when switched to the main lobe MLβ to enhance the accuracy of the movement direction detection.
Reducing the number of antenna elements is most effective for miniaturization, where the vertical direction and the horizontal direction desirably have the same directivity from the standpoints of inventory management such as VMI (Vendor Managed Inventory) and physical distribution management. The vertical and horizontal (vertical and horizontal directions) directivities are thus satisfactory, and the minimum array antenna becomes an array antenna 201 including 2×2=4 elements (211a to 211d) of two elements in the horizontal direction (X-axis) and two elements in the vertical direction (Y-axis), as shown in FIG. 8(a).
However, the applicant found through experiments that a new problem arises if the number of antenna elements is 2×2=4 elements. The new problem includes the problems of a side lobe and a grating lobe. In other words, as shown in FIG. 8(b), when switched to the main lobe MLα, a side lobe SLα becomes too large (similarly, when switched to the main lobe MLβ, a side lobe SLβ becomes too large), and the accuracy of the movement direction detection degrades. As shown in FIG. 8(c), if the side lobe becomes too large, the side lobe SLα generated on the − side at the same time as the generation of the main lobe MLα on the + side when switched to the scan angle α (similarly, the side lobe SLβ generated on the + side at the same time as the generation of the main lobe MLβ on the − side when switched to the scan angle β) communicates with the RFID tag (not shown). It is apparent through the experiments that the slope of the linear approximation line cannot be obtained, and the accuracy of the movement direction detection significantly degrades as a result.
A power distribution ratio to each antenna element is generally changed as shown in FIG. 9 to reduce such a side lobe. In other words, high power is supplied to the antenna element 212c at the middle and the power is lowered towards the ends in the plurality of antenna elements (212a to 212e). However, the control is complicating in such a method.
Patent Document 1: Japanese Unexamined Patent Publication No. 2006-20083
Patent Document 2: Japanese Unexamined Patent Publication No. 2007-303935